The present invention relates to a system and also to a computer program for the determination of quantities relating to the circulatory system of a patient, and especially for determining the circulating volume of blood and quantities derived therefrom by means of pulse spectrophotometric methods.
A system suitable for determining the circulating volume of blood is known, inter alia, from German patent document DE 41 30 931 Al. This is based on the principle of pulse spectrophotometry. Following the injection of a dye-bolus into the blood circulation system of a patient, the variation in the concentration of dye with respect to time is measured optically. This can be effected invasively by means of a fibre optic catheter or non-invasively by means of a reflection or transmission measurement at a finger, earlobe etc. A circulation transport function is then determined and a circulation response pulse that would correspond to an ideal indicator injection at a time point t=0 is calculated therefrom. A virtual dye concentration at the time point t=0 is then calculated by backward extrapolation of the circulation response pulse. The circulating volume of blood is given by the quotient of the amount of dye injected and the virtual dye concentration at the time point t=0. In the case of an invasive measurement, use is made of a fibre optic catheter which is technically relatively complex and expensive and, in addition, the application thereof imposes further stress on the patient. In the case of a non-invasive measurement, the problem of lack of precision in the measurement arises as will be discussed hereinbelow.
For the purposes of determining a plurality of quantities relating to the circulatory system, inter alia, the volume of blood that is circulating and the cardiac output CO by means of pulse spectrophotometric methods, it is necessary for the variation in the absolute concentration of the dye injected into the circulation to be known as precisely as possible, a qualitative measure for the variation of dye concentration over time is not sufficient. However, since the optical behaviour of biological tissue is mainly determined by the scattering of light so that the Lambert-Beer's law is no longer applicable, absolute concentrations cannot be determined by means of a non-invasive spectrophotometric measurement. A set-up for solving this problem is known from EP 0 505 918, wherein the ratio of the concentration of the injected dye to the concentration of a reference dye in the form of the haemoglobin that is always uniformly present in the blood and absorbs to a maximum extent in a differing wavelength range is determined. To this end, the extinction at the respective absorption maxima is determined by means of measurements at two different wavelengths. The absolute concentration of the haemoglobin in a blood sample is measured in vitro. A calibration is thereby effected, but this does not represent an online-calibration due to the in vitro measurement.
Other conventional non-invasive pulse spectrophotometric systems for the measurement of the circulating volume of blood, such as the system disclosed in U.S. Pat. No. 5,999,841, dispense with a calibration based on a blood sample and in vitro measurement of the haemoglobin concentration. However, the evaluating algorithms, which are based essentially on Lambert-Beer's law, cannot always ensure sufficient accuracy for the determined variations over time of the dye concentration and the quantities relating to the circulatory system derived therefrom due to the abovementioned scattering effects of biological tissue.